1. Field of Art
This invention relates to adaptive moving target indicator radar processing, and more particularly to improvements in discrimination against non-clutter-like signals in clutter phase estimation processes.
2. Description of the Prior Art
As is known, clutter-locking MTI radars enhance the detectability of moving targets in a clutter background by means of a clutter rejection filter, the return signals being processed therethrough after rotation in phase angle so as to shift the clutter phase centroid to the null of the MTI filter. The success of clutter rejection is commonly referred to as MTI improvement, and relates to the extent to which the clutter is suppressed with respect to moving targets. MTI processing known to the prior art generally provides an average of the phase difference, from one pulse repetition interval (PRI) to the next, of return signals in non-target range bins in the range vicinity of the particular signal being processed. This average phase difference is used to rotate the return signals, thereby concentrating the clutter near the null of the clutter rejection filter, so as to achieve maximum improvement.
Major practical problems in achieving maximum MTI improvement in radar systems known to the art have included the gross errors in estimated clutter phase which result by providing phase difference averaging in the angle domain, which does not compensate for discontinuity in angle and therefore provides significantly erroneous results in summations thereof; and in phase averaging which includes target return signal magnitude. These problems are largely overcome by the improved AMTI disclosed in the aforementioned copending application, in which phase averaging is achieved in the vector format, utilizing normalized sines and cosines of the return signal vectors, so as to avoid ambiguity and to avoid contamination from return signal magnitude variations. The utilization of unit vector, or normalized values of in-phase and quadrature components of return signals in the phase estimation technique tends to minimize corruption of estimated phase by inclusion of large targets therein; but, when targets become very large, such as spanning four range bins or the like, even a normalized representation thereof constitutes a significant component of the signal and results in large errors in average clutter phase estimation. For example, theoretical losses in MTI improvement which result from contamination by moving targets entering into the average phase difference calculation are reduced from on the order of 30dB by about 1.5dB for moving targest occupying a single range bin and by about 5dB for moving targets occupying four range bins.
One consequence of contamination of phase estimates is the occurrence of false alarms (that is, acquiring an indication of a moving target when there is none). This results from the fact that if the null of the clutter cancelling filter is adjusted at other than the true average clutter phase, then some of the clutter will survive the filter with a target-like magnitude and be recognized as a target, when in fact it is not. And, commensurately, actual moving targets which should be detected may be lost.